Christopher Meloni Calls Longtime Pal Mariska Hargitay by Pet Name in Kooky Birthday Tribute

Evans Vestal Ward/NBC/Getty

Mariska Hargitay celebrated her birthday on Monday, and at least one of her Law & Order: Special Victims Unit costars weren't going to let the occasion slip by unnoticed.

In a wacky tribute posted to Instagram, Christopher Meloni pulled so close to the camera that only his eyes and forehead were visible, and then sent a message to Hargitay, 59.

"Marsha," he hissed, referring to the actress by the pet name he uses. "Do you know what today is?"

He then leaned toward the camera and whispered, "I do."

In the caption of the post, Meloni, 61, added, "Celebrate it Marsha"

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Hargitay previously explained during a joint appearance with Meloni on Late Night With Seth Meyers last March that "Marsha" is "one of the many names" her costar has dubbed her with.

The duo also go by "Marshmelon", a combination of Marsha and Meloni, as well as "Bensler," a combination of her SVU character's last name, Benson, and Meloni's character's last name, Stabler.

Hargitay has starred in Law & Order: Special Victims Unit as Detective Olivia Benson since its premiere in 1999. Meloni played her partner Detective Elliott Stabler until his departure from the show in 2011. He returned to the franchise last year, reprising his role with his own spin-off series Law & Order: Organized Crime.

The two actors share an easy camaraderie that has charmed fans both on and off screen — so much that Hargitay even ended up listed as Meloni's wife on his Wikipedia page, despite them both being married to other people.

"I mean, I don't think I'll be marrying him any time soon, but I am kind of like his second wife," Hargitay explained to PEOPLE in September. "Wife, therapist, bestie, sister, mother."

Meloni similarly acknowledged "the fandom goes nuts" over his and Hargitay's onscreen chemistry, before wryly deadpanning: "I think the uber fans literally want our spouses to go away."

Will Hart/NBC Christopher Meloni and Mariska Hargitay

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In July, Meloni celebrated 27 years of marriage with his wife Sherman Williams. Meanwhile, Hargitay met husband Peter Hermann on the set of Law & Order: SVU in the early 2000s, and they marked 18 years of marriage in August.

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Hartigay's birthday comes just one day after Meloni celebrated yet another important birthday: His wife's 63rd. To mark the occasion, the actor shared an Instagram post of Williams dancing the night away in celebration.

"So…this happened. My gurl. Ageless. Timeless. Steppin' out with the young-uns to show em how it's done," Meloni captioned his post, adding the hashtags "#HBDBaby" and "#TalkAboutIt."